Heat staking as a means of joining parts together is well known and has been used in a variety of applications including the assembly of parts for the toy, medical and electronic industries. In general, one of the parts to be assembled is designed to include a plastic post which can be inserted through a hole or aperture in another part. The post is then permanently deformed by the generation or application of heat by some tool surface.
Ultrasonic welding is one method which is often used to deform these posts. The ultrasonic welding process produces heat by vibration. A welding tip or bell rubs against the post at a rate of more than 20,000 times/second thereby creating heat. With the appropriate combination of weld power, time and pressure this process can be used to join a variety of plastic parts, for example, the two halves of an audio cassette. When the vibration stops, melting within the weld joint ceases immediately. An ultrasonic staker is described in U.S. Pat. No. 4,859,378.
For ultrasonic welding to work, the parts must be clamped tightly to maintain alignment while the parts are subject to the vibrations of the welding process. Effective Welding can only occur if the tip can rub against the part. If the part is loose and free to move, welding will not occur. However, clamping the parts too tightly may cause damage (deformation) to the parts. Moreover, the vibrations themselves may cause misalignment of parts.
The ultrasonic welding process also tends to produce weld caps of varying diameter and shape because the plastic material is "free to flow". This leads to variable forces between the parts joined together. This is particularly critical when the parts to be joined are used to make precision electrical components such as switches. An example is an edge line microwave switch wherein a metal contactor is joined to a plastic slider. Making contactor assemblies for these switches with consistent bending forces is critical to ensuring switch reliability and is important to ensuring switch isolation performance. As a further concern with switch performance is the problem of glass fibers breaking off the stake and then interfering with electrical contacts.
Ultrasonic welding further tends to produce welds which withstand axial forces moderately well but do not tend to withstand torque very well. Thus it may be difficult to pull the two pieces apart yet be very easy to rotate the pieces about the stake. Further, weak or loose welds may result from variations in "stud" volume due to plastic molding changes or variations in depression volume due to tip wear from abrasion.
Use of heated tips is another method for welding two parts together. U.S. Pat. No. 4,767,298 discloses a device for heat staking plastic parts using this method. The device features a number of heating elements and individually programmable tips. In operation the tips are urged against the plastic posts, causing the posts to melt. The melted plastic is then free to flow as gravity and other forces dictate. This leads to weld caps which vary in size and strength. Allowing the plastic to free flow will not provide an assembly in which all of the glass fibers are contained in the weld cap thereby permitting the fibers to break off. Further, a high temperature may be required to achieve proper plastic reflow, but this high temperature may in fact degrade the performance characteristics of the material.
In U.S. Pat. No. 4,633,559, LOREN discloses a method and apparatus for staking a formable plastic post. The device relies upon physical force to form the plastic post into a desired shape. This is accomplished by clamping the device around the base of the post and thereby forming a cylindrical cavity around the post, after which a staking tool is driven into the post until the post fills the cavity. This method forms weld caps that are generally uniform in terms of size and shape, but which may vary in strength. Further, weld caps produced in this manner inherently have glass fibers which may break off and disrupt the workings of other parts due to the fibers not being contained in the weld.
In view of the above, it is an object of the present invention to provide a method and apparatus for staking in which uniform weld caps are produced.
It is another object of the present invention to provide staked weld caps which have a high resistance to torque and have few if any glass fibers to break off.